Technical Abstract:
Aquatic vegetation occurring in wetland habitats can be managed to mitigate agricultural contaminants exiting row-crop fields during storm events. Most such vegetation occurs as a mixture of different species and there is a need to better understand how multiple species can be managed to improve mitigation efficiency. In this study, combinations of two aquatic macrophytes, parrot feather (Myriophyllum aquaticum) and cattail (Typha latifolia), were used to assess mitigation of aqueous concentrations and effects on permethrin and nitrogen. Using hydraulically connected wetland microcosms (~50 L) in series, both species were tested singly (upstream microcosms Myriophyllum only, M; Typha only, T) and in combination (upstream to downstream Myriophyllum into Myriophyllum, MM; Typha into Myriophyllum, TM) to examine the influence of varying species along a linear hydraulic flow path. Upstream microcosms were dosed with a mixture of nitrogen (as ammonium nitrate) and permethrin simulating agricultural runoff. Aqueous concentrations of permethrin, NH4-N, and NO3-N were measured at 0 h (pre-treatment) through 8 h and again at 24, 48, 72, 96, and 168 h. Effects were assessed using Hyalella azteca 48 h aqueous laboratory bioassays at 0, 4, 24, and 48 h. Results showed permethrin was rapidly removed from the water column within 24 h. Mean permethrin half-lives (t1/2) ranged from 3-7 h with TM having a significantly (p < 0.05) longer t1/2 (6.8 h) than other treatments. Permethrin t1/2 was correlated (r = -0.5256, p < 0.05) with overall macrophyte biomass. Nitrogen as NH4-N and NO3-N steadily decreased over the 168 h study periods. Mean nitrogen t1/2 ranged from 59-238 h and 72-150 h for NH4-N and NO3-N, respectively, with T most rapid and M the least. Nitrogen t1/2 was also correlated (NH4-N: r = -0.8649, p < 0.01; NO3-N: r = -0.7859, p < 0.01) with overall macrophyte biomass. Effects of permethrin and nitrogen mixture toxicity occurred within 4 h and were mitigated within 24 h. Using the toxic unit model approach, the primary source of observed toxicity was permethrin. Despite apparent differences by aquatic macrophyte type, observed differences were due to overall differences in macrophyte biomass. As a result, overall macrophyte biomass appears to be the most important constituent in mitigating permethrin and nitrogen.